Separation of solid particles of various sizes from viscous liquids

The present invention relates to a process for separating solid particles of various sizes from viscous liquids, for example biomass from liquids of biotechnological processes, by pressure filtration.

In the workup of viscous polysaccharide solutions from biotechnological processes, a fundamental problem has to be overcome, namely to separate the biomass from the viscous liquid phase. In the case of the polysaccharide, a B-D-1,3-glucan, formed by fungus of strain ATCC 15205, there are additional problems due to the fact that a not inconsiderable proportion of the product adheres to the mycelium and is structurally similar to cell wall constituents.

The raw culture broth has hitherto been worked up by strongly diluting it, then filtering the dilute culture broth through conventional filter material, or centrifuging it, and reconcentrating the filtrate. In this sequence of operations, the dilution step is indispensable since the undiluted culture broth (an inhomogeneous mixture of mycelium, polysaccharide, media constituents and water) very quickly blocks up any conventional filter medium ever investigated or, owing to the high viscosity, renders sedimentation of the biomass through centrifugation impossible. Nor is it possible to separate the culture broth in a two-phase system owing to the structural similarity between the product and the cell wall.

Despite dilution, the lifetimes of membranes employed in the filtrations mentioned are only short. Nor does the prior art disclose a more effective process without higher hardware requirements.

It is an object of the present invention, starting from the prior art as represented in Ullmann's Encyclpadie der Technischen Chemie, 4th edition, volume 2, pages 156/157, to separate the biomass from ideally undiluted, raw culture broths.

We have fond that this object is achieved according to the invention by subjecting the inhomogeneous or homogenized suspension, without or only after a little dilution, to a pressure filtration whereby the solid particles of various sizes are retained according to size in a plurality of in particular in from 1 to 3, superposed or juxtaposed filter stages on filter layers of various depths and on filter surfaces and are separated from the viscous liquid.

Further features of the invention form the subject-matter of the subclaims.

In this pressure filtration, the pressure is exerted on the gas phase above the liquid to be filtered or directly on the liquid phase and is in the range from 1 to 10 bar. The filter layers for separating off coarse particles consist of a three-dimensional network of metal wire or synthetic cloth or cellulose material having an average pore width of from 1 to 3 mm and of sufficient depth (from 10 to 100 mm). For example, the density of such a synthetic filter is up to 80 kg/m 3 , while that of the metal version is correspondingly higher.

To separate off relatively fine particles the next stage comprises for example an open-pored polyurethane foam (average pore width from 0.2 to 0.8 mm, density up to 80 kg/m 3 ), followed finally for example by silk gauze (mesh size from 0.006 to 0.2 mm) or similar filter surfaces made of natural or synthetic materials. The pore width of the foam can be reduced in size by compressing the foam.

This filter technique is a combination of depth and surface filtration. In the upper filter layers first encountered by the flowing medium, the biomass is retained inside the filter, while at the silk gauze and similar filter cloths the pretreated suspension is subjected to a surface filtration.

The filter apparatus should be constructed in such a way as to maximize the size of the filter area. The flow on the inside must be influenced by constructional measures in such a way that the suspension cannot escape from passing through the filter layers.

This system is flexible and, by proper selection of the filter layers, is adaptable to different problem situations. The system has proved to be of good utility in particular in the case of highly inhomogeneous culture broths of high mycelium content (up to 10 g of dry matter/l) and high polymer concentrations (up to 12 g of dry matter/l) as obtained from shaking flask cultivations.

It will be readily understood that the process can also find utility in other fields where viscous liquids are worked up, as for example in the

production of celluloid

production of cellulose ethers

production of cellulosic fibers

production of fats and oils

production of gelatin

production of natural and synthetic resins

production of adhesives and sealants

production of pectin

production of starch

production of lubricants

production of other polysaccharides (agar, alginate, carrageen, dextran).

In what follows, the process according to the invention is further illustrated by reference to a drawing and to various Examples.

The drawing shows a section through a laboratory pressure filter unit used for filtering the various culture broths.

In the pressure filter unit, three different filter layers 1, 2 and 3 are arranged one below the other in the direction of flow or in certain circumstances even side by side. As in existing apparatus these filter layers are arranged on top of a filter support unit 4, into which position they are inserted from below. A coupling ring 5 connects the conical filter outlet 6 to the pressure filter unit and at the same time ties the filter support unit firmly into the pressure filter unit. The culture broth is fed in via line 7 and the filtrate is discharged via line 8. Line 9 is used to supply air or inert gas to apply the necessary pressure on the gas phase above the culture broth or directly on the culture broth. The pressure filter construction described permits rapid and easy replacement of the filter layers; however, this is also true of existing apparatus and not relevant to the invention. Conveniently, two pressure filter units are disposed in a parallel arrangement, so that there is always one unit in operation while at the same time in the other unit the filter layers are cleaned or replaced. It is also technically advantageous to clean the filter stages by back flushing.